page and attachment URL might conflict

[LukasLt]

Creating a page and attaching an image with exact same file name would create 2 different resources, that are expected to be available at the exact same URL (using url rewriting).

This leads attachment page being opened instead of the expected page (the attachment is uploaded later thus fetched prior the page)

E.g.
Create a page called Test. It would be published as site.com/test

upload featured image Test.jpg. The featured image attachment page would get a permalink of site.com/test

Expected result:
The attachment permalink has copy number appended like site.com/test-2

[SergeyBiryukov]

#15665